Both integrated circuits, as well as circuit boards (hereinafter referred to collectively as "electronic devices"), are commonly subjected to thermal shock testing during design qualification. Such thermal shock testing is typically accomplished by placing an electronic device in a metallic basket (e.g., stainless steel) that is alternately immersed in hot and cold liquid baths that circulate constantly to assure a uniform temperature in each bath. The hot bath is usually more than 130.degree. C. whereas the cold bath is at least -60.degree. C. To provide a robust test for thermal shock, the basket is transported between the hot and cold baths for 15 to 100 cycles, with a dwell time in each bath of approximately five minutes.
In practice, the hot and cold baths are filled with a fully-fluorinated liquid hydrocarbon, such as FLUORINERT liquid available from 3M Company, St. Paul, Minn. When the cold bath is filled with such a fluorinated hydrocarbon liquid, a static electric potential as much as 10,000 volts may be generated within an inch or so from the surface of the cold bath upon the immersion of the device. Such a potential will damage the electronic device in the basket, which is undesirable.
Past attempts at eliminating the excess charge have included reducing the turbulence within the cold bath by blocking one or more inlets through which the liquid is circulated. However, reducing the flow of liquid can damage the pump that circulates the liquid within the bath. Rather than block some of the inlets, a smaller size pump could be used. However, the disadvantage with using a smaller size pump is that such a pump may not sufficiently circulate the fluid to achieve a fairly uniform bath temperature. Other attempts at eliminating ESD have included ionizing the bath by way of a high-voltage supply. Although moderately effective, ionizing the bath generally entails extensive modification to the equipment employed to carry out thermal shock testing.
Thus, there is a need for a technique for mitigating ESD during thermal shock testing by alternately immersing a device in hot and cold liquid baths.